Around 12:13 Central Time on October 25, 1999, a Learjet 35 operated by Sunjet Aviation crashed in South Dakota, killing the captain, first officer, and four passengers.

The Learjet 35 is a business jet equipped with twin turbofan engines capable of carrying 8 passengers. The accident aircraft was manufactured in 1976 and was purchased by Sunjet Aviation in January 1999.

"NTSB-N47BA-slide0013 background" by NTSB - http://www.ntsb.gov/Events/2000/aberdeen/Opening%20Presentation.htm. Licensed under Public domain via Wikimedia Commons.

The accident aircraft departed from Orlando, Florida at 9:20 Eastern Time, destined for Dallas, Texas. The conversation between Air Traffic Control (ATC) and the pilots at that time was as follows:

At 09:21:46, the female first officer contacted Jacksonhill Control, “Passing 2900 meters, Climbing to 4300 meters.” At 09:21:51, ATC cleared them to climb to 7000 meters (approx FL230), and the crew acknowledged. At 09:23:16, ATC cleared the flight direct to Dallas, and the crew acknowledged. At 09:26:48, ATC contacted the crew for a frequency change, and the crew acknowledged. At 09:27:10, the first officer contacted ATC, “Climbing to 7000 meters.” At 09:27:13, ATC cleared the crew to climb to 11900 meters (approx FL390), and the crew acknowledged. (Radio communication with the aircraft was completely lost from this point until the crash.) At 09:33:38, ATC attempted to contact the crew for a frequency change, but received no answer. Over the next 4 minutes and 30 seconds, ATC made 5 successive calls to the aircraft, but received no answer.

When the pressurization system in the cockpit or cabin malfunctions, an automatic alarm sounds if the internal pressure drops below the equivalent of an altitude of 3000 meters. However, no cabin altitude warning sound was heard on the ATC recording.

We know that the amount of oxygen in the air is proportional to air pressure. For example, at an altitude of 10,000 meters, the air pressure is only one-fourth that of the ground. If the cabin is not pressurized and oxygen levels decrease, a person will fall into a coma and eventually die painlessly. Even if death does not occur, damaged brain function will not recover.

At 10:52 Eastern Time (09:52 Central Time), an F-16 fighter from Eglin Air Force Base in northwest Florida was radar-guided towards the accident aircraft. At this time, the Learjet 35 was flying at an altitude of 14,900 meters. At 10:10 Central Time, the F-16 pilot approached the accident aircraft. The aircraft showed no signs of external damage, no icing on the fuselage, both engines were operating normally, and the control surfaces were not moving. The cabin windows were dark, and the interior could not be seen. The cockpit right front windshield appeared to have condensation or ice on the inside; the left side was similarly opaque, appearing to be covered with a layer of thin ice. At 10:12, the fighter departed from the accident aircraft.

At 11:13, two F-16 fighters from the Oklahoma Air National Guard were radar-guided to the accident aircraft. Minnesota ATC reported that no figures were visible moving inside the accident aircraft, the windshield was dark, and no icing phenomenon was observed. At 11:33, one of the two F-16s flew ahead of the accident aircraft, but it elicited no reaction. At 11:39, the two F-16s departed from the accident aircraft.

At 11:50, two F-16s from the North Dakota Air National Guard were radar-guided to the accident aircraft. The two F-16s from Oklahoma returned again after aerial refueling, and four fighters flew together around the accident aircraft. At 11:57, the flight lead of the Oklahoma formation reported that the cockpit windows appeared to be covered in ice and all control surfaces were inactive.

At 12:10:41, the flight attitude of the accident aircraft began to change. According to the analysis of the Cockpit Voice Recorder (CVR) recovered later, the engine RPM began to drop at this moment, followed immediately by the recording of a Stall warning and the automated sound of the autopilot mode being disengaged. Additionally, during the 30 minutes of recording left on the CVR, the cabin altitude warning (indicating pressure altitude below 3000 meters) sounded continuously until 12:12:26.

At 12:11:01, according to ATC radar records, the aircraft began banking to the right and descending gradually. One North Dakota F-16 remained in the airspace west of the aircraft, while another returning from the tanker followed the accident aircraft as it continued to descend.

At 12:11:26, the flight lead of the North Dakota formation reported that the accident aircraft was continuously rolling, appearing to have lost control… descending very severely, requesting an emergency descent to continue tracking. An Oklahoma fighter also reported that the aircraft was about to hit the ground and was spiraling down.

In the image below, the green line represents the planned route, and the red line represents the actual flight path.

"NTSB-N47BA-slide0046 image002" by NTSB - http://www.ntsb.gov/Events/2000/aberdeen/Opening%20Presentation.htm. Licensed under Public domain via Wikimedia Commons.

This concludes the record of the accident. Below is the analysis of the cause.

The last contact between the crew and ATC was at 09:27:18, when the first officer acknowledged the instruction to climb to 11,900 meters. The aircraft’s altitude at that time was 7,070 meters. According to the subsequent investigation, the first officer’s speech showed no abnormalities, and there was no indication that she was wearing an oxygen mask. At 09:33:38, the aircraft became unresponsive; at this time, the altitude was 11,100 meters. Afterwards, the aircraft deviated from its cruising altitude of 11,900 meters and continued to Climb to 14,900 meters. Therefore, it is confirmed that in the 6 minutes and 20 seconds from 09:27:18 to 09:33:38, an anomaly occurred that deprived the pilots of their ability to control the aircraft. From the warning sound on the CVR mentioned above, we know that decompression had already begun in the early stage of the flight. Thus, the sole reason for the crew’s incapacitation was cabin decompression, and it is推测 that the crew members were not inhaling oxygen.

The flow control valve of the air conditioning system was recovered from the wreckage, and it was found that the valve was in the closed position. This valve controls the flow of pressurization and warm air; therefore, in the closed state, it is predicted that the cockpit and cabin experienced rapid decompression. If the valve were open, it would supply warm air into the cabin. This explains the icing phenomenon on the cockpit windows observed by the fighter pilots.

There are several possibilities for why the flow control valve was closed. One is a mechanical failure, such as a malfunction in the pressure measuring device or the valve actuator. Another is that the pilots did not turn the “Cabin Air” switch to “NORM” (Normal) before takeoff, meaning they left it in the “OFF” position. However, since the warning system activates when the cabin pressure altitude drops below 3000 meters, and oxygen masks automatically drop after 4300 meters, the accident report considers this possibility to be relatively small. Another possibility is that the pilots turned the “Cabin Air” switch off during flight.

As for why the crew did not use oxygen, the report suggests that the crew members may have already fallen into hypoxia before they could use the oxygen masks. In terms of possibilities, for example, if a crack occurred in the fuselage, it could result in either gradual decompression or rapid decompression. If rapid decompression occurs at an altitude of 9000 meters, cognitive function is impaired after 8 seconds without emergency oxygen, making complex aircraft manipulation actions impossible. In the case of gradual decompression, cognitive decline and loss of manipulation ability occur 30 seconds after the warning.

However, during the subsequent accident simulation, it was discovered that the content recorded in the Quick Reference Handbook (QRH) might have triggered the accident. This is because the QRH procedure for decompression first requires checking the status of the flow control valve (reportedly involving 4 lines of complex instructions), followed by putting on the oxygen mask. One can imagine that when experiencing hypoxia, the pilots were already in a state of reduced cognitive ability, and reading the difficult instructions in the QRH manual would certainly take time, causing the pilots to miss the final critical opportunity for emergency oxygen.

However, regarding the true cause of the accident, the report found no further evidence, so it remains unsolved.

References 1 http://en.wikipedia.org/wiki/1999_South_Dakota_Learjet_crash 2 まさかの墜落